Television signals based on present day standards create visible artifacts on conventional television receivers and monitors. In broadcast systems such as the 525 line-per-frame, 30 frame-per-second 525/30 NTSC system or the 625/25 PAL system, artifacts occur because of the interlaced scanning process. This process divides the 525-line picture or frame of the NTSC standard into two successive 2621/2-line fields. The 2621/2 lines of one field are scanned in 1/60th of a second followed by scanning of an additional 2621/2 lines of another field with lines of the second field occupying the spaces between the lines that of the first field. One subjective effect of this interlaced scan is to create in the presence of motion in the picture scene an apparent vertical drift of the lines of the raster known as "line crawl". The apparent drift is substantially more apparent when viewing a wide-screen display at close range. Another well-known objectionable visible effect is interline flicker on line-to-line transistion that occurs because of the interlaced scanning format.
Recent interest in the development of high definition television systems (HDTV) has been directed towards techniques that are intended to enhance the subjective performance of present systems within the contraints of existing standards. One approach, a technique referred to as progressive scan, or non-interlaced scan, has been described in patent materials and literature. For example, all the scan lines are scanned consecutively from the top of the display screen and downward to the bottom of the display screen during each vertical scanning interval. Progressive scan results in the reduction of interlaced scanning format related artifacts such as interline flicker and line crawl that exist in conventional two-to-one interlaced displays. The subjective effect is a more pleasing picture presentation to the viewer.
An aspect of the invention is using a field store and comb filters to generate from an incoming interlaced television signal two sets of interleaving display lines of a non-interlaced television display. In such an arrangement, in accordance with the invention, vertical detail is inserted in both sets of interleaving display lines.
Another aspect of the invention is that the picture details displayed in the non-interlaced format are made to substantially conform to the picture details contained in the interlaced signal, in the case where no motion in the picture scene is detected. Motion related aspects of the invention are described later on. Where no motion is detected, the picture information of one field of the interlaced NTSC signal is displayed in one set of the alternate scan lines, and that of the other field is displayed in a different set of scan lines that interleaves with the scan lines to form a picture frame. In the non-interlaced scanning format, each scan line of the one set is scanned immediately after the scanning of a scan line of the different set, for example, such that the picture progressively fill the screen of the display. All the scan lines of a picture frame are scanned consecutively during one vertical scanning interval.
Differences in scene content due to object movement or television camera panning that occur from one field time to the immediately following one are characterized as being visual motion of the scene content. If such motion is not taken into account in the course of displaying the picture contents of an interlaced signal in a non-interlaced scanning format, edges of moving objects may appear as having discontinuous transitions. This is so because the one set of displayed lines represents picture information that occurred one sixtieth of a second apart from picture information that is represented by the other set of displayed lines. The result is a jagged appearance for horizontal motion, and a smearing effect for vertical motion; such artifacts are objectionable.
Still another aspect of the invention is that when motion is detected in a section of the picture, then one field of the interlaced signal primarily is made to provide picture information for display in the scan lines of that section of the display where motion occurs. Thus, the picture information of moving edges of an object that is displayed in a non-interlaced format, is derived predominantly from one field of the interlaced signal. Therefore, the zigzag or jagged types of artifacts of moving objects that occur when a motion compensation scheme is not utilized, are reduced when the the picture information of moving objects is derived from one field of the interlaced signal.
Though some picture details are lost in such sections of the display screen when motion is detected, as a result of the motion compensation scheme of the invention, this loss in the presence of motion is not highly discernible. This is so because of the well-known fact that the ability of the human eye to discern fine picture details of moving objects is not as good as its ability to discern fine picture details of the same objects when they are stationary.